The present invention pertains to the bleaching of household linen in a household wash cycle, comprising a prewash, followed by rinsing and washing operations.
It constitutes, more particularly, an improvement of the process described in U.S. patent application Ser. No. 799,027 in the sense that a new means is placed at the industry's disposal to achieve and surpass the intended bleaching effect.
The persalts, particularly sodium perborate, are the bleaching agents most widely used in cleaning, but they are also rather uneconomical carriers of active oxygen.
Moreover, at temperatures below ca. 70.degree. C., these persalts or peroxyhydrates have sufficient activity only in the presence of so-called activating compounds, which considerably increase the cost of the compositions with bleaching action which contain them with or without detergents.
The process proposed in the above-referenced patent application permits the bleaching effect of such compositions to be improved.
In a household wash cycle comprising prewash in an alkaline bath with pH values between 9 and 13 at a temperature at most equaling 70.degree. C., in the presence of at least one peroxide bleaching agent, it consists of carrying out the said prewash at a temperature between 40.degree. C. and 70.degree. C. in the presence of uncomplexed calcium taken in a quantity between 0.002 and 1 wt.% relative to the weight of the bath and the subsequent conventional rinse and wash operations of the known cycles.
The presence of calcium in the prewash bath can be ensured, e.g., by dissolving calcium oxide (CaO) or calcium hydroxide (Ca(OH).sub.2) in said prewash bath.
It is preferably ensured by adding to the prewash bath a calcium salt whose anion is inert with respect to the peroxide bleaching agent and whose dissociation constant is higher than 0.01.
For example, calcium chloride corresponds to this definition particularly well.
The preferred quantity of uncomplexed calcium is between 0.005% and 0.1% of the bath weight.
The peroxide bleaching agent present in the prewash bath is selected from among those commonly used in the bleaching baths for bleachable stains of household linen or in the so-called complete detergents which assume the same function, such as, e.g., hydrogen peroxide, sodium perborate, sodium percarbonate and urea peroxyhydrate. The active oxygen concentration resulting from the presence of such bleaching agents is generally between 0.004% and 0.03% of the weight of the bath.
The prewash bath may contain complexing agents of metal ions, especially alkaline-earth metals, but doing so in such a quantity that the quantity of uncomplexed calcium characterizing the said bath is respect in the bath.
The prewash bath may also contain, in nature and in quantity, the detergent products and the other ingredients usually present in a standard prewash starting a known household wash cycle. Finally, the prewash bath may also be formed by a mixture; in an aqueous medium, of the alkaline agent, the peroxide bleaching agent and the salt selected as the source of calcium.
In such a case, the prewash bath preferably contains the calcium and the alkaline agent at a ratio that would correspond to the formation of Ca(OH).sub.2.
The preferred alkaline agent is sodium hydroxide (NaOH), but the product which is used as the calcium source may also be the sole or partial source of the alkalinity of the bath if it is selected from among calcium oxide or calcium hydroxide.
The preferred pH range is 10.5-12.5.
The temperature range of 40.degree. C.-70.degree. C. is preferred for carrying out the present invention, because it only leads to an insignificant improvement in bleaching effect at temperatures lower than 40.degree. C., while the energy cost soon becomes prohibitive at temperatures above 70.degree. C.
The duration of the prewash is basically dependent on the other conditions selected for this operation. For this invention, it is analogous to that of a classical prewash, i e., it is generally shorter than one hour.